The invention relates to a heating device having a disc-shaped heating element consisting of PTC thermistor material having a cut-off control property.
Heating devices have been described in prior art publications for about 2 decades, which have among other things, a disc-shaped heating element consisting of PTC thermistor material. PTC thermistor material relates to ceramic material on the basis of barium titanate having electrical conductivity with a sudden rise of the specific impedance with the so-called Curie temperature. See U.S. Pat. No. 3,441,517. This sudden behavior of the material having a positive temperature coefficient (PTC) was the cause for numerous suggestions to build heating devices with PTC thermistor material, which material - specifically and without the use of additional galvanic control contacts - has a cut-off control property. The cut-off control property is to be understood as a behavior in which the electrical impedance rise of the PTC thermistor material (having the Curie temperature) when heating the PTC thermistor material to temperature values higher than Curie temperature results, in a considerable decrease in the electric current flowing through the material. This can be equated with a reduction of the Joule heat U. I produced in the PTC thermistor material, and is thus equal to a decrease of heat output. The use of PTC thermistor material for heating devices such as, for example, in a solder gun (German Pat. No. 1,690,621) has been repeatedly suggested.
In spite of the numerous and not at all speculative suggestions of the technical world, and although suitable material had also been available for at least more than a decade, and even in spite of continuous demand for heating devices which have an absolutely safe automatic cutoff against overheating, no suggestions have been realized for heating devices having a greater specific heat output than 50 watts per cm.sup.2 of the cross-section of a PTC thermistor material disc.